A heat sensitive recording method has many advantages in that (1) no particular developing step is required, (2) if paper is used as a support, a recording material prepared can have a quality akin to that of plain paper, (3) handling of a recording material used is easy, (4) images recorded has high color density, (5) this method can be embodied using a simple and cheap apparatus, (6) no noise is generated upon recording, and so on. Therefore, heat sensitive recording materials have recently enjoyed a markedly increasing demand, particularly in the fields of facsimile and printer, and have come to be used for many purposes.
With this background, it has come to be desired to develop transparent heat sensitive recording materials which enables direct recording with a thermal head in order to adapt them for multicolor development, or to make them usable for an overhead projector (hereafter it is written as OHP).
However, conventional transparent heat sensitive recording materials are so-called transparent heat sensitive films in which the film is brought into direct contact with an original and exposed to light, and thereby an infrared portion of the light is absorbed by image areas of the original to raise the temperature of the image areas, which results in color development of the heat sensitive film. Accordingly, they do not have heat sensitivity high enough to enable direct heat recording with a thermal head to be used in facsimile and the like.
In addition, a heat sensitive layer of heat sensitive recording materials of the kind which use a thermal head upon heat recording is in a devitrified condition, so a desired transparency cannot be achieved by merely coating such a layer on a transparent support.
As heat sensitive recording materials which can solve the above mentioned defects, we had suggested a new transparent heat sensitive recording material which can be prepared by using a combination of a colorless or light colored electron donating dye precursor and a color developer as color development system, wherein the former is microencapsulated and the latter is emulsified and dispersed under a certain condition, then both are mixed and coated on a support.
However, from the heat sensitive layer of above mentioned transparent heat sensitive recording material, the color developer is apt to come out to deposit. Therefore, the transparency of the heat sensitive recording material is lowered and an extent of it depends strongly on a stability of an emulsified dispersion which is utilized when the heat sensitive recording layer is coated and also depends on a kind of the color developer.
As a result of concentrating our energies on solving above mentioned defects, we have found that it is quite effective to use a partially saponified polyvinyl alcohol for a protective colloid which is used in the before mentioned emulsified dispersion, and/or to use a metal salt of salicylic acid derivative together with the color developer to prevent the deposition of the color developer from the heat sensitive layer, then we have completed the present invention.